TOMOGRAPHIC RECONSTRUCTION OF 3-D SEISMIC VELOCITY AND ATTENUATION STRUCTURE BENEATH MERAPI VOLCANO AND ITS SURROUNDINGS

• Main Author: RAMDHAN NIM: 32413300, MOHAMAD
• Format: Dissertations
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/29000
• Institution: Institut Teknologi Bandung

Mount Merapi is a stratovolcano that is one of the most active volcanoes in the world. Many earth scientists have conducted research on this volcano using various methods. The geological features around the volcano are also very attractive to be studied because they were formed from the complex tectonic process and volcanic activities for tens of millions of years ago. Southern mountain, Kendeng basin and Opak active fault are such features caused by the compound process and activities. <br /> <br /> <br /> A project called DOMERAPI was conducted e.g. to understand magma reservoirs of the Merapi volcano comprehensively. The DOMERAPI network was running from October 2013 to mid-April 2015 by deploying 46 broad-band seismometers around the volcano. The BMKG data were used jointly with the DOMERAPI data to determine hypocenters of earthquakes simultaneously recorded by both networks, because the majority of earthquakes occured outside the DOMERAPI network. The BMKG network was useful to minimize the azimuthal gap from the epicenters. The ray paths from these distant earhquakes could be used to delineate the depth of magma bodies beneath Merapi volcano. <br /> <br /> <br /> In this study, hypocenter relocation was conducted before travel-time seismic tomographic investigation. There are 399 events from 464 earthquakes that were successfully relocated from the two seismic networks. The hypocenter relocation was conducted in order to obtain precise hypocenter locations. The result of this relocation clearly depicts e.g. backthrust structure to the South of Java and a cluster of events to the east of the Opak fault suggesting that the fault has an eastward dip. <br /> <br /> <br /> Tomograms of Vp, Vp/Vs and Vs from both networks successfully detect a shallow and an intermediate magma bodies or reservoirs at depths of around 5 km and 15 km beneath the Merapi volcano. The shallow magma reservoir is related to low Vp, high Vp/Vs and low Vs, which may associate with molten material. The intermediate magma reservoir is related to high Vp and Vp/Vs and low Vs, which may associate with fluid filled cracks. The zone between the two magma bodies has lower Vp/Vs compared to those of the two magma bodies. The zone is related to impermeable zone between the two magma reservoirs. This zone is most likely related to the magma supply trajectory that has cooled down since the 2010 eruption. High Vp anomaly at the intermediate reservoir depicts that no hot material suply from the deep magma reservoir to the intermediate magma reservoir. This is one of the causes of Merapi that has not erupted since 2010. Here, we note that the existence of the deep magma reservoir identified by the petrology study is not well identified in the velocity tomogram resulting from this study. To the north of Merapi, high Vp, low Vp/Vs and high Vs beneath Merbabu volcano are detected clearly. These velocities indicate that the activity of the volcano is much lower compared to the activity of Merapi. In addition, the southern mountain zone to the south of Merapi volcano is detected clearly as a zone with high Vp, low Vp/Vs and Vs anomalies. <br /> <br /> <br /> Travel-time seismic tomographic inversions using data from four seismic networks (DOMERAPI, MERAMEX, BMKG and BPPTKG) were also carried out. The resulting tomograms depict in general similar anomalies as revealed by tomograms from the two (DOMERAPI and BMKG) seismic networks for the depth less than 30 km. The MERAMEX network data enhance the resolution of images of the deeper area. The partial melting zone beneath the study area is identified clearly at the depth of around 120 km. The DOMERAPI network contribution is very significant in increasing the image resolution for intermediate and shallow structures. The velocity anomalies less than 30 km depth mentioned above could not be detected clearly without the DOMERAPI data. <br /> <br /> <br /> The magma of Merapi volcano can also be detected by the Q tomogram of P wave (Qp). The Qp values beneath Merapi volcano are much lower than those beneath Merbabu volcano. The existence of the shallow and deep intermediate bodies are detected clearly by the Qp tomogram. The shallow magma body lies at depths around 5 km; while the intermediate magma body exists at depths of more than 15 km. The anomaly between both zones shows a high Qp value. This anomaly may be related to the magma supply of the 2010 big erruption. At present the magma supply zone has cooled down and produced a high Qp anomaly. <br />